Olefinic hydrocarbon monomers for the synthesis of conventional mass polymers such as polyethylene and polypropylene are typically obtained by a steam cracking process.
In contrast, fluorinated olefin monomers such as tetrafluoroethylene (TFE) and hexafluoropropylene (HFP) as well as vinylidene fluoride or vinyl fluoride, which are required for the production of partially and fully fluorinated (perfluorinated) polymers (fluoropolymers) are currently only accessible via an intricate multi-stage process on industrial scale. Said process involves chlorine chemistry, and it requires rather high amounts of energy which is a drawback with regard to environmental as well as economical aspects. Further, numerous chlorinated intermediates and side-products are generated, and an expensive disposal of large quantities of aqueous hydrochloric acid waste contaminated with hydrofluoric acid is required. Additionally, in many countries around the world, regulatory bodies do not allow TFE to be transported due to its tendency to explode spontaneously, which requires a direct further processing at the production site.
Due to their exceptional properties, fluoropolymers cannot be substituted in high-tech applications such as in the semiconductor sector, in electrical/electronic systems, as seals and corrosion-resistant material in the field of environmental protection, and in technologies of energy conversion, e.g., fuel cells. Fluoropolymers allow continuous operating temperatures of above 280° C., are typically non-flammable, exhibit a high dielectric strength and low dielectric loss, good mechanical and sliding properties, inert behavior, and biocompatibility.
The state of the art process for producing TFE and HFP uses chlorodifluoromethane (R-22) as an intermediate and is based on chlorine, methane, and hydrofluoric acid. In a first step, trichloromethane as well as numerous undesired side-products are obtained by partial chlorination of methane, the trichloromethane then being further processed with hydrofluoric acid in the presence of an antimony chloride catalyst to yield chlorodifluoromethane. The latter compound is pyrolyzed in a third step at 800° C. to 900° C., and difluorocarbene obtained via elimination as an intermediate leads through subsequent dimerization to TFE. The process yields hydrochloric acid, different chloromethanes, antimony fluorides, and fluoromethanes. The R-22 pyrolysis is primarily carried out in tubular reactors only in the gas phase. Due to the endothermic reaction of about 64 KJ/mol, a temperature gradient occurs within the used tubular reactors, which results in a decrease of conversion and selectivity. Therefore, the yields in the reactors are only 30-40%, which, however, is intended, since at low conversions the selectivity to TFE is very high, and the formation of side-products can be reduced. However, the TFE synthesis through the R-22 route yields a large number of waste materials, which have to be processed, separated, and typically thermally recycled.
Therefore, new processes and reactors are needed which enable the production of the desired monomers, such as TFE and HFP, based on less reactive, chlorine-free starting materials, in particular from at least partially fluorinated or even perfluorinated paraffins, which, in turn, can be obtained on an industrial scale by electrochemical fluorination. The development of such new processes is required since the resulting fluoropolymers are of high importance for the production of special plastics as key components in the chemical technology, biotechnology, automotive and electronics industry. Further, the large amounts of aqueous hydrochloric acid waste being contaminated with hydrofluoric acid should be reduced or avoided.
As an alternative TFE synthesis based on the decomposition of fluorinated materials processes with high energy impact such as plasma or arc have been reported for example in U.S. Pat. Nos. 2,902,521, 2,709,182, 3,133,871, 2,709,192, 3,133,871, 3,009,966, 3,471,546, 3,904,501, 4,849,554, 4,898,645, 4,973,773, 5,611,896, WO 99/59385, U.S. Pat. Nos. 6,624,337, 7,622,693 and 6,919,015.
The fluorinated compounds to be decomposed to create fluoroalkenes can be obtained, for example, by electrochemical fluorination, starting from hydrofluoric acid and short-chain aliphatic compounds as known in the art. The processes for a chlorine-free TFE synthesis described above, however, have so far been of limited economical interest. Another route has been described in WO2010/039820 where fluorinated material is subjected to microwave irradiation in a fluidized bed reactor. However, there remains the need for a further process for producing fluorinated alkenes and an apparatus for such a process allows a synthesis with satisfying conversions and yields as a replacing for the R-22 route.